During the manufacture and forming of many products from sheets or webs of plastic material, thermal-forming machines are used to simultaneously mold large quantities of plastic thin-walled articles. A typical molded article is formed from one of a large variety of generally cup-shaped constructions, the article being formed between mating two-piece dies or molds suitable for imparting to the finished piece its final desired shape. A typical thermal-forming machine has a pair of mating male and female dies, or molds that are brought together on opposed sides of a pre-heated web of plastic material, during an operating cycle. Usually, a plurality of mating male and female dies are provided on bottom and top platens, or die carriers, respectively, enabling production of a plurality of articles during a single cycle of operation.
According to one set-up, a separate trim press machine is provided adjacent to the thermal forming machine for separating the plurality of molded articles from the web of plastic material. A typical machine trim press is set up adjacent to the output side of the thermal forming machine, where it operates on the web of plastic material to remove the molded articles immediately adjacent to the location where they have been formed. A typical trim press has a fixed lower platen and a reciprocating upper platen. Each platen is configured transverse to the path of travel of the web of plastic material, so that they come together on opposite sides of the web, while the web and the in-molded articles are held in an accurate fixed position between the platens. Complementary cutting surfaces are formed in the top and bottom platens in locations that severe the in-molded articles from the web of material as the platens close onto the web. Typically, the movable upper platen has a spring seated clamp that engages with the top of the web, forcing it into engagement on its bottom face with the lower platen. In this manner, the clamp locks the web into position over the lower platen, just prior to engagement of the cutting surfaces and severing of the web about each article. Alternatively, a spring seated stripper carried on the lower platen strips the web off the lower die, and furthermore, acts as a spring seated clamp which holds the web during severing.
Preferably, the lower platen is held in a fixed position, immediately beneath the web of material. In this manner, the lower platen also supports the web as it is fed into the trim press for a subsequent operating cycle. Typically, the web is fed into the trim press during the period of time that the upper platen is raised from the lower platen. As the upper platen is being lowered, the mechanism feeding the web is stopped at a desired location and the clamp (or stripper) further engages the web, fixing the web in an accurate location between the platens suitable to severe the articles therefrom.
Modern thermal forming machines have provided vast productivity improvements by increasing the rate with which articles can be produced from a single machine. Many of these machines are driven by one or more electric drive motors. Alternatively, hydraulic or pneumatic actuators can be used to impart motion to a thermal-forming machine. Additionally, a control system or even a complex arrangement of kinematic linkages can be configured to choreograph the associated movements of feeding, heating, and forming of plastic articles by the machine. In fact, the use of computers and high speed processing has enabled vast improvements in cycle speed for thermal-forming machines.
However, as the productivity of thermal-forming (thermoforming) machines has increased dramatically, trim presses have become the slow component of a forming and cutting operation, limiting the output of the entire line. State of the art trim presses need to more than double the existing maximum expected rate of 160 cycles per minute (cpm) to rates in excess of 300 cpm. Such presently unsuitable state of the art devices include mechanical product picking devices, and even servo motor driven feed mechanisms.
Therefore, improvements to trim presses are needed in order to enable the trimming of articles from a web, particularly during high speed thermal-forming, or molding operations. One problem results from high speed movement of the upper platen which shakes the trim press. As machine cycle speed increases, the dynamic forces created by the moving upper platen of the trim press greatly complicate the design of an accurate high speed trim press machine. Even where flywheels are added to the kinematic drive linkage on the press, oscillations can still occur in the rotational velocity of the flywheel. This can lead to jerky motion of the upper platen, resulting in poor high speed cutting performance. Therefore, improvements are needed to ensure accurate, uniform, and smooth closure between the top and the bottom platens of a trim press in order to ensure high speed and accurate cutting capabilities suitable to enable use of the trim press with a modern thermal-forming machine. Furthermore, improvements are needed to enhance cutting performance, by reducing imbalance forces created by the moving upper platen, while minimizing the required support structure of the machine.
Another problem results from the speed limitations imposed when using traditional servo motor driven feed wheels to feed the web of material into the trim press. As the servo speed approaches 200 revolutions per minute (rpm), the feed wheels on each edge of the web can actually rip the web because the web is not strong enough to overcome the weight of the material in the web. One prior art technique has involved the use of pairs of wheels on each edge of the web, one (a drive wheel) having a plurality of circumferentially spaced apart and radially extending picks which perforate the web along each edge, to engage the web and enable feeding there along, and the other acting as a follower wheel. However, such constructions tend to tear the web, and are not capable of producing speeds necessary to exceed 160 cycles per minute (cpm). In fact, to feed a web of material into a trim press that is running at 400 cpm, the servo motor and wheels will run at about 2,000 rpm. Therefore, additional improvements are needed in order to enable the feeding of a web of plastic material into an improved high speed trim press.
The objective of the present invention is to provide a vastly improved machine trim press having features for reducing the dynamic operating forces and to enable high speed feeding of a web of plastic material into the trim press. Furthermore, features are desired for offsetting undesired dynamic imbalance forces in an operating trim press while at the same time producing smooth axial cutting forces, resulting in precise and accurate cutting of articles from a web of material during a forming operation, such as a thermal-forming cycle of a thermal-forming machine.